The last common ancestor of Eugnathostomes
What kind of skeleton did the ancestral eugnathostome have? Did it have lots of dermal armor, little, or none?
We have four groups of well-known living and fossil gnathostomes to derive inferences from:
- Placoderm-grade vertebrates with heavy dermal bone in a unique pattern
- Chondrichthyes with tiny odontode scales
- Osteichthyes with a standard pattern of dermal bones - often extensive
- Acanthodii with little dermal bone
Placoderm grade vertebrates:These have plenty of dermal bone, but in a unique pattern that makes it impossible (?) to compare with that of Osteichthyes.
Early stem chondrichthyans:As we survey farther down the chondrichthyan stem, we find creatures that share chondrichthyan synapomorphies but increasingly share ancestral characters with other gnathostome groups. A sampling:
Ozarcus mapesae from Pradel et al. 2014.
Doliodus problematicus pectoral fin from Miller et al. 2003.
Gladbachus adentatus (Devonian) Little information in the literature, however in his 2013 podcast, Coates describes the results of his CT scans of Gladbachus, noting:
- The shape of the neurocranium is reminiscent of that of placoderm-grade gnathostomes
- The hyoid arch, although involved in jaw suspension, is not strongly differentiated from adjacent branchial arches
- The shape of the palatoquadrate suggests that at least some of the adductor muscle mass passed medially to it, as in placoderms!
A general pattern emerges in which, the farther down the chondrichthyan branch we push:
- The less clearly distinct animals become from osteichthyans
- The more placoderm-grade plesiomorphic features we recognize.
Climatius from Wikimedia.com
Acanthodii(Late Ordovician - Permian)
Remember these guys:
- Fin spines on all fins. (In some, intermediate fin spines develop from the ancestral posterior fin-fold. Also, cf. Doliodus, above.)
- Heterocercal tails
- Often lack teeth or, when they are present, lack evidence for regular replacement
- Anal fin supported by a spine (unique)
Climatius from Carroll, 1988
- Very heavy fin spines.
- Four pairs of intermediate spines between pectoral and pelvic fins.
- Multiple external gill slits, partially covered by a larger operculum. (Armored by branchiostegal rays.)
- anterior and posterior dorsal fin.
Acanthodes from Wikimedia.com
- very light fin spines
- one pair of intermediate spines
- Gills entirely covered by operculum.
- Anterior dorsal fin lost.
- Overall resembling a spiny version of the basal osteichthyan Cheirolepis
Difficulty: Generally speaking, one would expect that in an evolving monophyletic group, the more derived taxa would occur later in time than the more ancestral ones. This is called stratigraphic congruence. Acanthodians display the opposite pattern. The most freakishly spiny ones are also early.
Acanthodes neurocranium mandibular, and hyoid arches
- Placoderms are paraphyletic
- Acanthodians are paraphyletic, with:
- some (like Climatius) basal to Eugnathostomata
- some (like Acanthodes) closer to Osteichthyes
- some (like Kathemacanthus) closer to Chondrichthyes
- a body covered with a mosaic of small scales but no dermal bone plates
- a cartilagenous endoskeleton
And the people read it and rejoiced that the enigma had been resolved. But only until 2013.
The Source of All Disquiet
Then came Zhu et al. 2013, reporting on Entelognathus primordialis, a placoderm-like creature with extensive bony cranial and thoracic armor and:
- Osteichthyan - like dermal cranial elements: the premaxilla, dentary, and maxilla.
- Jaw adductor musculature on both the medial and lateral sides of the palatoquadrate.
Adding color to this pattern:
- Sahney and Wilson, 2001 discovered the described the remains of open endolymphatic ducts in the acanthodians Brochoadmones, Ischnacanthus, Lupopsyrus, and Tetanopsyrus - a feature shared with living chondrichthyans but not osteichthyans.
- Davis et al. 2012 redescribed the skull of Acanthodes - the most osteichthyan-like of acanthodians - and found it to be more chondrichthyan-like than previous describers had.
- Brazeau and de Winter, 2015 examine the skull of Acanthodes, revealing that it's hyomandibula articulates with the braincase ventrally to the jugular vein. In osteichthyes, this articulation either straddles the vein or is dorsal to it. We don't know which state is derived, but the similarity to living chondrichthyes is striking.